Twisting, broadening and stretching the capacities of the orchestral tuba are methods aptly demonstrated on this quartet of CDs, two solo and two duos. Although each is uniformly impressive, what is also notable is that the extended and microtonal strategies used by both German-based low-brass men were separately serendipitously developed.

Contemporary notated music ensembles are where Brighton, England-born Robin Hayward is usually employed, playing his own compositions and pieces composed for him by the likes of Alvin Lucier and Christian Wolff. Now Berlin-based and involved with ensembles such as the Splitter Orchestra and Phosphor, Hayward helped develop a microtonal tuba which uses an exchangeable vale system to extend the instrument’s range to play pitches without lip-bending. Divergently, Freiburg-born Carl Ludwig Hübsch’s background was initially in punk and brass bands. Now Köln-based however, his more recent improv credentials extend to groups featuring fellow sound explorers such as saxophonist Frank Gratkowski and trombonist Wolter Wierbos. Surprisingly or not, the restrained and somewhat other-worldly textures Hübsch creates solo on Die Sach an Sich and in his duo with Christoph Schiller, aren’t that dissimilar from those propelled by Hayward on States of Rushing and his duo with Roberto Fabbriciani

You get the clearest idea of his style on “Treader”. Here palindrome-like mouth smacks expand with staccato percussiveness to such an extent that the brass rumbles start to resemble conga drum whacks. Following a section where the bellowing echo back onto itself, with broken-octave patterns, tongue twists are replaced by forced breaths – and what could be light footfalls – as the slower pulse melds with brassy growls and ends with staccatissimo crunches. The subsequent “Redial” adds to this sonic picture with timbres so taut and microtonal that the effect suggested is that of plastic being flanged and physically pulled. Nevertheless the blurry oscillations include vocal inflections, so that the humanness of the performance – and performer – is never in doubt. As the exchangeable valve system makes it possible to alternate between standard and micro-tonal pulsing two separate lines are audible until a finale which collapses both into a heavily breathed timbre.

If Hayward’s solos reflect piping advances, then Hübsch’s cram an expansive collection of tones into their conception. More highly rhythmic and multiphonic than Hayward’s work, the German layers contrapuntal friction into many of his lines. For every time he allows pure air to echo through his instrument uninterrupted, there are interludes of strident tongue and lip motions that could be air leaking gradually from a balloon; rhythmic slaps and rubs on the metal surface; mouthpiece suction and aviary squeals; plus whistling semi tones and watery pumps.

On “Teil 4” he pumps out a swinging ostinato like a one-man New Orleans brass band, then turns out short chromatic melody inserts that contrast with rapid crackles and tongue vibrations. Like Hayward, he outputs pounding drags and rebounds, suggesting the sounds of a drum and cymbal. Unlike the Briton however, another intonation strategy appears to result from blowing through an aluminum pie plate balanced on his instrument’s upturned bell. Hübsch also alternates watery tremolo lines and plunger extensions. Meanwhile “Teil 2” varies continuous narrowly spaced tongue puffs, capillary brays and even drum stick-like slaps on the metal. His low-pitched snarls and gurgles also resonate back into the tuba bell. Soon the instrument’s elephantine bellow is spiced with tongue slurs and pops, as distant higher pitched tones are added to the constant drone.

More similarities between tubaists exist when another partner is added to the mix. That’s because both duos deepen the search for timbres far beyond the expected. Hayward’s associate Fabbriciani, who pushes the limits of his flute tones in a similar fashion to the tuba, spends most of the disc playing his self-designed, hyperbass flute with its more than 12 meters of tubing. Although this is the duo’s first improvised session, Hayward and Fabbriciani have together performed late works by Luigi Nono. Sharing similar interests in electronics and notated music, Hübsch and Stuttgart-born, Basel-based Christoph Schiller, master of the prepared and altered spinet, have improvised together since 2008. Although mostly involved with the improvising vocal ensemble Millefleurs, Schiller has also worked with players such as violinist Harald Kimmig and Peter Baumgartner on powerbook. His self-designed attachments convert the 16th Century keyboard into a string-percussion instrument.

This flexibility is quite obvious on Giles U, when his rasping take on tension-laden extensions push the strings firmly into bottleneck-guitar territory. Besides additional harsh twangs, Schiller also exposes keyboard plinks plus sawing tones resonating off tightly wound strings. At the same time, these timbres are frequently answered by, or contrast with, reflux cries, duck-like quacks and flutters, plus rough throat-clearing honks from Hübsch.

These knife-edge echoes are most in evidence on track 5, where the scrapped and stopped strings are pushed to such flanging that the result resembles hurdy-gurdy-like splintered tones. The tubaist’s contribution is in the form of rubato burbles, pedal-point slurs and corkscrew plunger work. Here and elsewhere lines evolve in double counterpoint, only occasionally intersecting.

It’s the same on track 2, although the tuba player’s interface is more concerned with distanced whistling, tongue slaps and alp-horn-like echoes. As Hübsch continuously growls in strained tones, Schiller’s responses take the form of cymbal-like key clatters, plus rolls, pops and strokes on the internal strings. The broken-octave playing is also more involved with color and shading then connection. However by the end the keyboardist’s exposure of string sounds quivering with partials and extensions, plus the tubaist’s solid yet guttural snorts, reach a common goal.

So do the metal-infused polytones Fabbriciani and Hayward both exhibit on Nella Basilica’s five Tuscany-recorded selections. However with both playing horns, the linkage of most tones to an individual instrument is more difficult than divining a spinet’s texture from a tuba’s. “Adagio” for instance, alternates lowering damp plops, contrapuntal quivering drones, tongue stops and portamento scrapes. By the end however identifiable tuba warbles are heard alongside melismatic counter tones from the flute. In a similar fashion, “Colori di Cimabue” consists of back-and-forth, expansive horn lines. Soon, cavernous tuba rumbles and bubbling meet microtonal key percussion plus discontinuous pumps and squeals. As chirping tones are matched with subterranean lowing, the revealed tones affiliate with human-sounding vocalizing through the different metals. True differentiation only occurs on pieces like the title track with the slightly higher pitched flute lines sounding airy and chromatic as the tuba tones corkscrew into pedal-point growls that never venture higher than mid-range. By the finale of this intermezzo, the flute whistles splutter in the background as bent-note tuba pedal point occupies the foreground.

Making the case for adventurous sound construction as well as the versatility of the hitherto lumbering tuba tones, Hübsch and Hayward showcase advances in their chosen instrument’s range and texture; plus confirming its role as a duo partner.